This Creative Commons-licensed piece first appeared at Climate News Network.

LONDON, 21 July, 2015 – Between 2007 and 2013 emissions of carbon dioxide from fossil fuels burnt in the US fell significantly ? by about 11% ? and many analysts credited this to a change from coal to natural gas in electricity production.

But new research says it was, in fact, the economic recession that explains most of the decline, and more extensive use of natural gas may not do much to slow global warming.

"Natural gas emits half as much CO2 as coal when used to make electricity," says research professor Laixiang Sun, of the Austria-based International Institute for Applied Systems Analysis (IIASA), who conducted the study with colleagues from the University of Maryland, US.

But that is only part of the story, he says in a paper published in the journal Nature Communications. "This calculation fails to take into account the release of methane from natural gas wells and pipelines, which also contributes to climate change."

Methane is 34 times more potent a greenhouse gas than CO2 over a century, but 84 times more over just 20 years.

In the US, coal-powered electricity fell from 50% to 37% of the generation mix between 2007 and 2012, with most of it replaced by natural gas ? in large part, due to fracking and underground mapping technologies.

"If we don't understand the factors that led to this emissions reduction, we won't know how to effectively reduce emissions in the future"

The researchers say that because this shift occurred at the same time as the reduction in emissions, many commentators linked the two, mistaking temporal co-incidence for causality.

Apart from the state of the economy, they say, there were several other factors involved: population growth, for example, and energy efficiency, which both also affect total emissions.